Nuclear medicine is a unique medical specialty wherein radiation is used to acquire images which show the function and anatomy of organs, bones or tissues of the body. Radiopharmaceuticals are introduced into the body, either by injection or ingestion, and are attracted to specific organs, bones or tissues of interest. Such radiopharmaceuticals produce gamma photon emissions which emanate from the body and are captured by a scintillation crystal, with which the photons interact to produce flashes of light or “events.” Events are detected by an array of photodetectors, such as photomultiplier tubes, and their spatial locations or positions are calculated and stored. In this way, an image of the organ or tissue under study is created from detection of the distribution of the radioisotopes in the body.
One particular nuclear medicine imaging technique is known as Positron Emission Tomography, or PET. PET is used to produce three-dimensional images for diagnosing the biochemistry or physiology in a specific organ, tumor or other metabolically active site. In PET events are detected from the decay or annihilation of positrons and electrons. When a positron is annihilated by an electron, two 511 keV gamma photons are simultaneously produced and travel in approximately opposite directions. Gamma photons produced by an annihilation event can be detected by a pair of oppositely disposed scintillating detectors capable of producing a signal in response to the interaction of the gamma photons with a crystal of the scintillating detectors. Annihilation events are typically identified by a time coincidence between the detection of the two 511 keV gamma photons in the two oppositely disposed detectors, i.e., the gamma photon emissions are detected virtually simultaneously by each detector. When two oppositely disposed gamma photons each strike an oppositely disposed detector to produce a time coincidence, they also identify a line of response, or LOR, along which the annihilation event has occurred. An example of a PET method and apparatus is described in U.S. Pat. No. 6,858,847, which patent is incorporated herein by reference in its entirety.
The number of time coincidences detected within a field of view (FOV) of a detector is the count rate of the detector. The count rate at each of two oppositely disposed detectors, A and B, can be referred to as singles counts, or singles, SA and SB. The time required for a gamma photon to travel from its point of origin to a point of detection can be referred to as the time of flight, or TOF, of the gamma photon. TOF is dependent upon the speed of light c and the distance traveled.
A time coincidence, or coincidence event, is identified if the time difference between the arrival of signals in a pair of oppositely disposed detectors is less than a time coincidence window τ. The number of coincidence events per second registered is commonly referred to as prompt coincidences P, or prompts. Prompt coincidences P include true coincidences T, or trues and random coincidences R, or randoms. True coincidences T are those physically correlated time coincidences, i.e., two gamma photons emitted in the process of annihilation or photons produced from the two primary gamma photons. Random coincidences R are those time coincidences that are not correlated, but randomly occur within τ. The randoms from a pair of detectors are usually proportional to the time coincidence window τ according to the following formula: R=τ·SA·SB.
In the case of a system composed of a plurality of detectors, the total randoms can be estimated as R=k·τ·S2, where S is the average single count rate per detector and k is a proportionality constant. The prompt coincidences P, which are used to reconstruct an image of the distribution of activity in the patient, are therefore the sum of true and random coincidences, (P=T+R). The presence and detection of randoms is problematic as they degrade the quality of the image and limit the rate of true data throughput. Consequently, given data transmission limits, the rejection of randoms at their origin increases the count rate capabilities for true coincidences. The primary method for reducing the number, or fraction, of randoms in the data stream has been to reduce the time coincidence window τ. However, two factors have limited the reduction of the time coincidence window τ; the time resolution of the system and the size of the field of view (FOV). While the time resolution of PET systems is consistently improving as a result of improvements in detector technology and electronics, the time coincidence window τ is generally limited by the size of the FOV, i.e. the time window τ must be large enough to accept coincidence photons from anywhere in the FOV, as explained in the following.
As illustrated in FIG. 1, if an annihilation event occurs at the center of a FOV, the TOF of the gamma photon detected in detector A (TA) is equal to the time of flight of the gamma photon detected in detector B (TB). If an annihilation event occurs at a distance Δx from the center of the FOV, the difference between TA and TB is Δt=2Δx/c, where c is the speed of light. If r is the radius of the FOV, the TOF difference Δt could take any value from −2r/c to +2r/c, depending on the location of the event. Because a source, or location, of an annihilation event is unknown, a priori, the time coincidence window τ must be great enough to accept all true coincidence events occurring within the FOV, so τ must be greater than 4r/c. The number of randoms that a conventional PET system can reject is, thus, limited by the required size of the time coincidence window τ.
What is needed then is a method for performing PET such that randoms may be rejected beyond the above described time coincidence window limits.